This invention relates generally to superconductive articles. More particularly, this invention relates to an article having a substrate, a thermochemically stable, amorphous layer comprising tantalum or a tantalum-containing material, a layer of material having a rock salt-like structure that has substantial alignment both in-plane and out-of-plane, a superconducting layer formed proximate said rock salt-like layer that has substantial in-plane and out-of-plane alignment, wherein said rock salt-like layer provides a template for the epitaxial growth of said superconducting layer, and an optional buffer layer or layers with substantial alignment both in-plane and out-of-plane, between the rock salt-like structure layer and the superconducting layer.
Current research in the area of high temperature superconductivity includes the use of biaxially textured high temperature superconducting thin films. Most such high temperature superconducting thin films have been grown on single crystal substrates, which promote the growth of oriented epitaxial films. However, such single crystal substrates may not be suitable for all applications.
High temperature superconductive articles include, at least, a substrate and a superconducting layer (such as YBa2Cu3O7xe2x88x92x (xe2x80x9cYBCOxe2x80x9d)) deposited thereon. One or more other layers are generally included between the substrate and the superconducting material. The purposes of these other layers are varied, but are, for example, to provide a biaxially textured template, to reduce or eliminate negative reactions between the superconducting material and the substrate and/or the template layer, and/or to protect another layer or layers of the article from adverse results of exposure to conditions, such as high temperature, that may be used to apply or treat subsequently applied layers. Ultimately, a crystallographically ordered superconductive layer is desired that can carry large currents without resistive loss.
Ion beam assisted deposition (IBAD) is used to produce biaxially textured films on non-epitaxial substrates that can subsequently be used as a structural template for the deposition of other films, thereby eliminating the requirement for single crystalline, biaxial substrates.
Yttria-stabilized zirconia (YSZ) can be applied to a substrate using IBAD, and provides a desired biaxial texture with desired crystallographic orientation. However, the desired texture of YSZ evolves slowly, so a thick film or layer is required to achieve good in-plane alignment. Due to its high processing time and cost, the application of YSZ to a substrate is not favored.
Materials that are characterized by rock salt-like structures, due to their simple crystal lattice, can be applied to crystallographically disordered substrates using IBAD to provide a good template for superconducting layers. These materials may be applied in thin layers, allowing them to be applied more quickly than YSZ.
Rock salt-like materials need a non-crystalline (amorphous) surface upon which to be coated to result in a coating having the preferred orientation. If the surface upon which the rock salt-like material is coated is not amorphous, but is polycrystalline or some other structure, the rock salt-like material will be predisposed to have the same structure as that surface and may result in a poor template upon which to ultimately coat a superconducting layer.
WO 99/25908 describes coating a layer of rock salt-like material on a substrate having an amorphous surface provided by a coating of Si3N4 or SiO2 on the substrate.
The use of Si3N4 or SiO2 as an amorphous layer in high temperature superconductive articles, however, is problematic. For example, the silicon atoms of Si3N4 or SiO2 diffuse through subsequent buffer layers that are applied over the Si3N4 or SiO2 and into the superconducting layer, and reduce the conductivity of the superconducting layer.
Thus, the need still exists for a substrate with an amorphous layer upon which a rock salt-like material, optional subsequent buffer layers, and, ultimately, a superconducting layer may be applied, and which results in the superconducting layer having good critical current capability.
The inventors recognized that a thermochemically stable, amorphous layer comprising tantalum or a tantalum-containing material coated on a polycrystalline substrate could act as a smoothing layer on the substrate that would provide an amorphous surface upon which to establish a rock salt-like material layer with a preferred biaxial texture. The rock salt-like layer then could serve as the template for epitaxial growth of subsequent layers, including a superconducting layer, which would ultimately have the desired orientation to result in good critical current densities.
The amorphous layer comprising tantalum or a tantalum-containing material also acts as a barrier between a metallic substrate and subsequently applied layers. The layer is a barrier specifically to cations that may migrate from the substrate to the rock salt-like layer, and that may affect the crystallographic orientation of the rock salt-like layer, which would ultimately negatively affect the critical current density of the superconducting layer.
The inventors also recognized that since tantalum has an extremely slow diffusion rate through some other materials and since tantalum ions are relatively large in size, cations of tantalum would not migrate in sufficient amounts to adversely affect the crystallographic ordering of the rock salt-like material that is applied over the layer comprising tantalum or a tantalum-containing material nor would they significantly diffuse into the superconducting layer to adversely affect the conductivity of the superconducting layer.
The present invention provides an article comprising: a) a substrate having at least one major surface; b) a thermochemically stable, amorphous layer comprising tantalum or a tantalum-containing material, wherein said thermochemically stable, amorphous layer is formed on at least one major surface of said substrate; c) a layer of material having a rock salt-like structure formed on said thermochemically stable, amorphous layer and that has substantial alignment both in-plane and out-of-plane; and d) a superconducting layer having substantial in-plane and out-of-plane alignment that is formed proximate said rock salt-like layer; wherein said rock salt-like layer provides a template for epitaxial growth of said superconducting layer.
The article may further comprise at least one buffer layer between said rock-salt like layer and said superconducting layer, wherein said at least one buffer layer substantially replicates the template provided by said rock salt-like layer.
The article may further comprise a protective layer applied on said superconducting layer.
As used herein:
xe2x80x9cAmorphousxe2x80x9d means substantially lacking crystalline structure as indicated by X-ray diffraction;
xe2x80x9cBiaxial texturexe2x80x9d means that the crystallites in a layer are substantially aligned with both a direction perpendicular to the surface of the layer and a direction in the plane of the layer;
xe2x80x9cBiaxial layerxe2x80x9d means a layer of material having biaxial texture;
xe2x80x9cCrystallitexe2x80x9d means that portion of a crystal whose constituent atoms, ions, or molecules form a perfect lattice, without strains or other imperfections. Single crystals may be quite large, but crystallites are usually microscopic;
xe2x80x9cEpitaxial growthxe2x80x9d means that the crystallographic orientation of the layers applied subsequent to a biaxial layer (the rock salt-like layer in this case) is derived from and directly relates to the crystallographic orientation of the biaxial layer and/or of the intermediate layer that is based on the template provided by the biaxial layer;
xe2x80x9cIn-planexe2x80x9d describes the direction that is substantially parallel to the plane of the article or layer;
xe2x80x9cOut-of-planexe2x80x9d describes the direction that is substantially perpendicular to the plane of the article or layer;
xe2x80x9cRock salt-likexe2x80x9d refers to a crystalline structure with atomic arrangements identical to or similar to that in rock salt, such as cubic structures with (100) close-packed planes or cubic structures in which a part of the lattice (some planes) have the same atomic arrangement as in rock salt, and means a structure having a cF8 Pearson symbol and a B1 Sturrukturbericht designation; and,
xe2x80x9cThermochemically stablexe2x80x9d means that the material or layer of material itself does not substantially react with adjacent or nearby materials or layers, and that it is stable to temperatures of up to about 950 degrees Celsius.